This invention relates to hydrothermal oxidation processes and to equipment for facilitating hydrothermal oxidation reactions. More particularly, the invention relates to an arrangement for bringing an oxidant into contact with a reactant material to oxidize the reactant material in a hydrothermal process. The invention encompasses both hydrothermal treatment systems and methods.
Hydrothermal oxidation involves bringing a reactant material to be oxidized, water, and an oxidant together under an elevated temperature and pressure to effect a partial or complete oxidation of the reactant material. Hydrothermal processes may be carried out at various combinations of temperature and pressure. For example, the reaction temperature may be below the critical temperature for water and the pressure may be below the critical pressure for water. Alternatively, the temperature or pressure, or both may be at or above the respective critical point for water. Although the critical temperature and pressure may vary somewhat depending upon other materials present with water, the critical temperature for water is approximately 705 degrees Fahrenheit and the critical pressure is approximately 3200 pounds per square inch.
Hydrothermal processes may be employed in many different applications. For example, hydrothermal processes may be used to treat wastewater containing organic and inorganic contaminants. In particular, municipal and industrial sewage sludge may be destroyed using a hydrothermal process to produce primarily heat energy, clean water, carbon dioxide gas, and residual minerals and salts. Heat energy from the hydrothermal process may be used to generate electricity. Also, organic fuels such as coal or petroleum may be oxidized in a hydrothermal process to produce heat energy that can be used for electrical power generation.
Hydrothermal oxidation occurring at conditions above both the critical temperature and pressure for water is commonly referred to as supercritical water oxidation or SCWO. Water at supercritical conditions (SCW) is neither a liquid nor a vapor, but can be properly characterized only as a supercritical fluid having a density significantly less than liquid water but significantly greater than water vapor. The density of SCW increases with increasing pressure at constant temperature. At very high pressures, greater than 40,000 psia for example, the density of SCW resembles that of liquid water. For the purposes of this discussion, SCW is assumed to exist at moderate temperatures between 705xc2x0 F. and 1200xc2x0 F. and moderate pressures of 3200-5000 psia. Nonpolar substances such as oxygen gas and most organic compounds are highly soluble in SCW. Due to the solubility of organic compounds and oxygen in SCW and the characterization of SCW as neither a liquid nor gas, SCW provides essentially a single-phase reaction environment that eliminates the relatively slow process of transferring reactants and products between separate gas and liquid phases. The single-phase reaction environment combined with a high reaction temperature in SCWO results in rapid and complete oxidation of organic compounds. Thus, it is desirable in a hydrothermal process to conduct at least part of the reaction at supercritical conditions in order to rapidly and more completely oxidize the given reactant material.
While nonpolar substances such as oxygen and most organic compounds are highly soluble in SCW, polar substances that may be encountered in hydrothermal processes have very low solubility in SCW. In particular, inorganic compounds such as salts have very limited solubility in SCW even though they may be very soluble in liquid water. Typically, the solubility of salt in water changes by relatively small amounts as the aqueous solution is heated. The solubility change may be seen as a slight increase or decrease in the solubility limit, depending on the specific salt. If the solution is heated to its critical temperature, the solubility of the salt will experience a sudden decrease as the water transitions from a polar solvent to a nonpolar solvent. The largest reduction in salt solubility generally occurs in the near-critical temperature range of 650xc2x0 F. to 720xc2x0 F. The temperature at which a given salt in an aqueous solution begins to experience the sudden decrease in solubility will be referred to in this disclosure and the accompanying claims as the xe2x80x9csalt precipitation temperature.xe2x80x9d
Although the solvent properties of SCW are very desirable in destroying organic compounds, the low solubility of inorganic salts in SCW has posed problems in prior SCWO systems. Salts may enter a SCWO system as part of the feed stream being treated or may form later in the reaction stream as a result of hydrolysis and the oxidation of organic heteroatoms such as sulfur, phosphorus, and nitrogen. Regardless of the source of the salts in the SCWO system, the salts precipitate from the reaction stream as the salt precipitation temperature of the solution is approached. The precipitated salts adhere to the internal surfaces of devices in the SCWO system to form scale. These scale deposits may occur in heat exchangers, heater coils, and reactors in a SCWO system, resulting in reduced heat exchange capacity, increased back pressure within the system, and ultimately, a completely plugged system. Thus, SCWO systems must be shut down periodically to remove scale deposits and thereby restore heat transfer efficiency and prevent plugging.
Numerous solutions have been proposed to overcome the salt scaling problem in hydrothermal processes. Some proposals include arrangements that treat rapid scale formation as an inevitability, and simply address the cleaning process. Other proposed solutions involve protecting the walls of the hydrothermal reactor to prevent deposition of precipitated materials. One of these wall-protecting solutions is described in U.S. Pat. No. 5,670,040, and involves conducting the supercritical oxidation reaction in a special platelet or transpiration tube. This transpiration tube includes openings that allow water to be continuously injected into the tube. The injected water is intended to form a protective barrier at the surface of the tube in order to prevent precipitated materials from adhering to the tube.
The previously proposed solutions to the scaling problem in hydrothermal processes, including the above described proposals, have generally proven unacceptable for various reasons. Most of the proposed solutions are costly and do not adapt themselves well to a continuously operating, robust system. Others simply do not work at the demanding conditions required for supercritical water oxidation. Considering the desirable attributes of supercritical water oxidation for waste treatment and other applications, there remains a need for a solution to the problem of rapid salt scaling in supercritical water oxidation systems.
The present invention provides hydrothermal treatment systems and methods that reduce or eliminate salt deposition (scaling) in the hydrothermal oxidation reactors, heat exchangers, heaters, and related equipment. A preferred hydrothermal treatment system according to the invention includes a contactor and reactor arrangement that receives a water stream, a first reactant material stream, and a second reactant material stream, with each stream pressurized to a processing pressure at or above the critical pressure for water. The contactor and reactor arrangement places the three input streams together so as to effect a hydrothermal reaction between the two reactant materials and thereby produce a hydrothermal reaction effluent.
As used in this document, xe2x80x9chydrothermal reactionxe2x80x9d or xe2x80x9chydrothermal oxidationxe2x80x9d means an oxidation reaction in the presence of water at an elevated temperature and pressure. A xe2x80x9cfirst reactant materialxe2x80x9d means the feed material to be treated in the hydrothermal oxidation process, and may include water along with materials to be oxidized. A xe2x80x9csecond reactant materialxe2x80x9d refers to a suitable oxidant that may be combined with the first reactant material together with water to effect the desired hydrothermal oxidation of the first reactant material. Such an xe2x80x9coxidantxe2x80x9d may comprise any oxidant material suitable for use in the hydrothermal oxidation process including, air, oxygen, hydrogen peroxide, or nitrate, for example. The phrase xe2x80x9creaction streamxe2x80x9d will be used herein to describe the stream of materials existing after oxidant, water, and reactant material are combined in the hydrothermal system, while the phrase xe2x80x9chydrothermal reaction effluentxe2x80x9d will be used to describe the stream of material resulting after the desired hydrothermal reactions have occurred.
In addition to the contactor and reactor arrangement, a hydrothermal treatment system according to the present invention further includes an effluent stream splitting arrangement for receiving the hydrothermal reaction effluent and splitting this effluent to produce two separate effluent streams. These separated effluent streams will be referred to herein as a xe2x80x9cfirst split effluentxe2x80x9d and a xe2x80x9csecond split effluent,xe2x80x9d and are used to heat at least the water and the first reactant material prior to being injected into the contactor and reactor arrangement. In a preferred form of the invention, a first split effluent heat exchange arrangement places the first split effluent stream in a heat exchange relationship with the water input to the contactor and reactor arrangement. A second split effluent heat exchange arrangement places the second split effluent stream in a heat exchange relationship with both the first reactant material and the water input to the contactor and reactor arrangement.
It will be appreciated that the various input streams to the contactor and reactor arrangement are carried through suitable conduits as are the hydrothermal reaction effluent and the two split effluent streams derived from the single hydrothermal reaction effluent stream. The contactor and reactor arrangement is connected to a water conduit, a first reactant material conduit, a second reactant material conduit, and a hydrothermal reaction effluent conduit. The effluent stream splitting arrangement preferably comprises a hydrocyclone having its input connected to the hydrothermal reaction effluent conduit, its overflow output connected to the second split effluent conduit, and its underflow connected to the first split effluent conduit. Each heat exchanger arrangement operatively connects one of the split effluent conduits to one of the input conduits to effect a heat transfer between the respective effluent carried through the respective split effluent conduit and the input material carried through the respective input conduit. As used in this disclosure and the accompanying claims the term xe2x80x9coperatively connectedxe2x80x9d when used to describe the relationship between two conduits and a heat exchanger means that the heat exchanger is connected to the conduits so as to allow materials from the conduits to flow into the heat exchanger and effect a transfer of heat between the materials.
The effluent stream splitting arrangement and heat exchanger arrangements according to the present invention enable the heat energy from the hydrothermal reaction effluent to be used efficiently to increase the temperature of the first reactant material and water to appropriate temperatures for conducting the hydrothermal reaction in the contactor and reactor arrangement. In particular, a preferred treatment method according to the present invention uses heat energy from both split effluent streams to heat the water input to the contactor and reactor arrangement to a temperature above the critical temperature for water. Heat from one of the split effluent streams is preferably used to heat the first reactant material to a temperature near the critical temperature for water but below a charring temperature for the first reactant material. For purposes of this disclosure and the accompanying claims, the xe2x80x9ccharring temperaturexe2x80x9d for a given reactant material is defined as the temperature at which the rate of formation of thermal decomposition products (char) in the reactant material results in unacceptable fouling of heat transfer surfaces in the hydrothermal treatment system.
These and other objects, advantages, and features of the invention will be apparent from the following description of the preferred embodiments, considered along with the accompanying drawings.